Suggestion: short-circuit type inference for ternary operators when both branches have the same type

From https://github.com/squidfunk/typescript-issue-38339-repro (courtesy of @squidfunk)

TypeScript Version: 3.9.1-rc

Search Terms:

Subtype reduction, ternary

Code

import { JSX } from "preact"

type IntrinsicElements = JSX.IntrinsicElements[keyof JSX.IntrinsicElements]

export type IntrinsicElements2<
  T = IntrinsicElements
> = {
  [K in keyof T]: K extends "accept"
    ? T[K] | [string]
    : T[K]
}

export type IntrinsicElements3<
  T = IntrinsicElements2
> = {
  [K in keyof T]: K extends "class"
    ? T[K] | [boolean]
    : T[K]
}

export type IntrinsicElements4<
  T = IntrinsicElements3
> = {
  [K in keyof T]: K extends "id"
    ? T[K] | [number]
    : T[K]
}

export function foo(): IntrinsicElements4 | undefined {
  let t: IntrinsicElements4 | undefined
  // this takes 2 seconds to check
  return t
  // this takes 10 seconds to check
  // return true ? t : t
}

Expected behavior: return t and return true ? t : t check in a comparable amount of time

Actual behavior: the ternary is much slower

Playground Link: https://www.typescriptlang.org/play/?ts=Nightly#code/JYWwDg9gTgLgBAbzgKQMoA04F84DMoQhwBEYUApgIYDGMxAUPTAJ5jlwCSAdjFMFwGdg1AKIAbciHI8BcALwoMAOm69+Q0RKkyA2gGtyzCLkXoVPPoOHjJ0mAIC6jcgA9IsOCzacL661rsBACYAHno4OAAVeR81K01bGXoAPhiEcLgdAGk4fjgDIxNIhwAuOBzXGGkAE1liGmpyMDoMiIB+KOyHOAAfTIE4gHMnCIiyyK76LGc3aHgvdlVLDRttewBmMIjohSW-BLXglLSM7NyufMNjKNLyuEqauuoxSgEBBlG4Doms7r6dABGEAgEkoXBGo3Gk2m9Fc7nmrEWvniq0CABYtlEYnsUQEZOtjgp0hEznkCtdimUKi4qlxaiRgNUPqNvl1epkuABXEAA8hQCFjTq-KYzeF4TlcWjACAXXDAgAUAEoyjiVnj7Gj2RLquRcPxyNVEBkJPMVci1YkNVq6br9dUMgB6B0UGCcqAXeBOuAAMVeMAyLrdHqgnPYHXgZU9DrgqDEEAA7iKgA

Related Issues: #38339

[ahejlsberg]

Also, the IntrinsicElementsX mapped types can be written more efficiently as:

type IntrinsicElements2 = IntrinsicElements |
  { [K in keyof IntrinsicElements]-?: K extends "accept" ? [string] : never };

type IntrinsicElements3 = IntrinsicElements2 |
  { [K in keyof IntrinsicElements]-?: K extends "class" ? [boolean] : never };

type IntrinsicElements4 = IntrinsicElements3 |
  { [K in keyof IntrinsicElements]-?: K extends "id" ? [number] : never };

This flattens the type into a union instead of stacking multiple mapped types on top of each other.

Even more efficient is to have a single type do the augmentation:

type IntrinsicElements4 = IntrinsicElements |
  { [K in keyof IntrinsicElements]-?:
    K extends "accept" ? [string] :
    K extends "class" ? [boolean] :
    K extends "id" ? [number] :
    never
  };

[amcasey]

@squidfunk We’ve been building popular OSS projects looking for perf issues, but it’s very valuable to have author input on intentions and expectations. Otherwise, it’s hard to for us to tell whether the performance we’re seeing is worse than it ought to be and whether the types can be changed without losing something important.